1. Field of the Invention
The present invention relates to an alternating current corona discharge apparatus. More specifically, the present invention relates to an alternating current corona discharge apparatus particularly suited for an electrophotographic apparatus.
2. Description of the Prior Art
A corona discharge apparatus comprises a thin wire electrode and a shield plate enclosing the wire electrode and a high voltage of the order of several kV is applied between the wire electrode and the shield plate. It has been observed that generally uniform ion discharge is achieved along the wire electrode on the occasion of the positive corona discharge where the positive high voltage is applied to the wire electrode whereas a glow spot is likely to occur on the wire electrode on the occasion of the negative corona discharge where the negative high voltage is applied to the wire electrode so that a uniform ion discharge is not achieved. Such trend is seen even in an alternating current corona discharge apparatus wherein the positive corona discharge and the negative corona discharge are repeated alternately for each half cycle and thus uniform ion discharge is difficult to attain in the negative half cycle corona discharge. When a charge is imparted onto a member being charged such as a photosensitive member of an electrophotographic apparatus as a result of such non-uniform ion discharge, then an electric charge on the surface of the member being charged becomes non-uniform.
In order to eliminate such uneveness of the surface charge on the member being charged caused as a result of the negative corona discharge, it has been proposed that the negative high voltage being applied to the wire electrode be increased in the negative direction to make more dense the glow spot and also to increase the corona current so that uneveness of the surface potential on the member being charged is reduced. However, such an approach for increasing the voltage being applied to the wire electrode involves an upper limit voltage by virtue of a fear of a spark discharge and leakage. In addition, it has been observed that an increase of the voltage being applied as the discharge voltage causes a high frequency component of a large peak value in superposition on each peak in the positive direction and in the negative direction of the waveform of the corona discharge current. Such phenomenon uniquely occurs when a corona discharge apparatus is used as a load. Such high frequency component becomes a cause of an unstable discharge and an increased discharge voltage considerably increases the instability of the discharge.